1. Field of the Invention
The invention relates to a mechanic drive apparatus, more particularly to a mechanic drive apparatus which is simple and compact in construction and which is capable of maintaining a fixed output speed for a specified operating load.
2. Description of the Related Art
Referring to FIG. 1, a mechanic drive apparatus which was disclosed by the applicant in his co-pending U.S. patent application Ser. No. 07/985,193, filed on Dec. 3, 1992, is shown to comprise a driving shaft 1, a driven gear 2, now U.S. Pat. No. 5,267,919, an intermediate transmission unit 4 and a set of worm segments 3. The driven gear 2 is mounted rotatably on the driving shaft 1. The intermediate transmission unit 4 includes a pair of intermediate shafts 41, a pair of driven transmission shafts 42 and a disc-shaped support 43. Each of the intermediate shafts 41 is disposed transverse to the driving shaft 1 and has one end mounted rotatably thereto. The other ends of the intermediate shafts 41 are secured to one end of a respective connecting member 403. Each of the driven transmission shafts 42 has one end mounted rotatably on the other end of the respective connecting member 403. A driving wheel 411 is mounted rotatably on each of the intermediate shafts 41 and has a circular rim which is in frictional contact with the support 43. Endless flexible bands 412 are provided to enable the driving wheels 411 to drive rotatably the driven transmission shafts 42. Each of the worm segments 3 meshes with the teeth of the driven gear 2 and is provided on the other end of a respective one of the driven transmission shafts 42. Each of a complementary pair of linkage members 44 is formed with a keyway 442 and has one end connected to the driving shaft 1 so that the linkage members 44 will rotate with the same. Each of the driving wheels 411 is provided with a key 405 which engages a respective one of the keyways 442. A coil spring 402 is disposed around each of the intermediate shafts 4 1 to bias the driving wheels 411 toward the axis of the driving shaft 1. Changes in the speed of the driven gear 2 relative to the driving shaft 1 occurs when the driving wheel 411 moves axially along the axis of the driven transmission shaft 42 so as to vary the position of frictional contact between the driving wheel 411 and the support 43.
Referring to FIG. 2, the linkage members 44 rotate with the driving shaft 1 when an external driving force is applied to the latter. The resulting centrifugal force will cause the driving wheels 411 to move along the intermediate shafts 41 until the centrifugal force matches the biasing force of the coil springs 402. At this stage, the intermediate shafts 41 and the driven transmission shafts 42 revolve around the axis of the driving shaft 1 due to the engagement between the keys 405 and the keyways 442. Since the driving wheels 411 are in frictional contact with the support 43, the driving wheels 411 rotate relative to the intermediate shafts 41 due to rotation of the latter relative to the support 43, thereby enabling the driving wheels 411 to drive rotatably the driven transmission shafts 42 by means of the endless flexible bands 412. The worm segments 3 rotate with the driven transmission shafts 42. Aside from driving rotatably the driven gear 2, slight shifting movement of the worm segments 3 relative to the driven gear 2 also occurs, thereby resulting in an output speed which is different from that of the driving shaft 1. The distance between the driving wheels 411 and the axis of the driving shaft 1 varies in accordance with the speed of the driving shaft 1. When the driving wheels 411 are disposed closer to the center of the support 43, the difference between the input and output speeds is relatively small. When the driving wheels 411 are disposed farther from the center of the support 43, the difference between the input and output speeds is larger. This indicates how the mechanic drive apparatus achieves stepless variable-speed control of the driven gear 2.
It is noted that since the above described mechanic drive apparatus is relatively large and requires a large number of components, the mechanic drive apparatus is not suitable for use in relatively small devices.